OCR A Level Biology:复习笔记2.6.3 Identifying Mitosis in Plant Cells

Identifying Mitosis in Plant Cells

  • Growth in plants occurs in specific regions called meristems
  • The root tip meristem can be used to study mitosis
  • The root tip meristem can be found just behind the protective root cap
  • In the root tip meristem, there is a zone of cell division that contains cells undergoing mitosis
  • Pre-prepared slides of root tips can be studied or temporary slides can be prepared using the squash technique (root tips are stained and then gently squashed, spreading the cells out into a thin sheet and allowing individual cells undergoing mitosis to be clearly seen)

Micrograph showing a stained root tip

Method

  • Garlic or onion (Allium cepa) root tips are most commonly used (the bulbs can be encouraged to grow roots by suspending them over water for a week or two)
  • Remove the tips of the roots (about 1cm) and place in a suitable stain (eg. warm, acidified acetic orcein, which stains chromosomes a deep purple)
  • The stained root tip is gently squashed on a glass slide using a blunt instrument (eg. the handle of a mounting needle)
  • Cells undergoing mitosis (similar to those in the images below) can be seen and drawn
  • Annotations can then be added to these drawings to show the different stages of mitosis

Analysis

Micrograph showing a cell undergoing prophase (P)

 

Micrograph showing cells undergoing metaphase (M) and anaphase (A)

 

Micrograph showing cells undergoing metaphase (M) and anaphase (A)

 

Micrograph showing a cell undergoing anaphase (A)

Limitations

  • It can be very difficult to distinguish between prophase and telophase in cells
    • Looking at the number of nuclei within one cell can help to identify which stage is likely to be occurring. If there are multiple nuclei within one cell then telophase is occurring.

     

  • The size of cells or structures of tissues may appear inconsistent in different specimen slides
  • Optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that can not be seen
  • The treatment of specimens when preparing slides could alter the structure of cells

Exam Tip

It is important to be able to recognise each mitotic stage from electron micrographs and to be able to explain why that cell is in the stage you have selected.

 

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